Imam al-Shafi'i (may Allah have mercy on him) said: "Among these (forms of testimony) is that which the witness has (himself) observed, so he testifies on the basis of observation."
The Shaykh (Imam al-Bayhaqi, may Allah have mercy on him) said: "What is meant by this are the actions that you observe and then testify to on the basis of observation."
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Isa bin Maryam (peace be upon him) saw a man stealing, so he said to him: 'Did you steal?' He said: 'No.' He said: 'By Allah, besides whom there is no god?' He said: 'By Allah, besides whom there is no god.' Then Isa (peace be upon him) said: 'I believe in Allah, and I belie my eyes.'"
Imam al-Shafi'i (may Allah have mercy on him) said: And among them (the forms of testimony) is that which is attested by widespread reports, most of which cannot be witnessed directly, while knowledge of it becomes firmly established in the hearts; so he testifies to it on this basis.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20581